Color filters are used in liquid crystal displays (LCDs), optical filters for cameras, and the like. The color filters may be fabricated by coating a fine region with three or more colors on a charge-coupled device or a transparent substrate. The colored thin film can be fabricated by dyeing, printing, electrophoretic deposition (EPD), pigment dispersion, inkjet printing, and the like.
The dyeing method forms a colored film by forming an image having a dyeing agent on a substrate and then dyeing the image with a direct dye and the like. Examples of the dyeing agent used in fabricating a colored thin film include natural photosensitive resins such as gelatin and the like, amine-modified polyvinyl alcohols, amine-modified acrylic-based resins, and the like. However, the dyeing process may be complex and lengthy, since it should include resist-printing whenever a color needs to be changed to form a multicolored thin film on the same substrate.
In addition, many generally-used dyes and resins may have good color vividness and dispersion but also poor color fastness, water resistance, and heat resistance, which are very important characteristics. For example, azo and azide compounds have been used as a dye but have deteriorated heat resistance and durability compared with a pigment.
The printing method forms a colored thin film by printing an ink prepared by dispersing a pigment into a thermally curable or photocurable resin and curing it with heat or light. This method may decrease a material cost compared with other methods, but it is difficult to form a fine and precise image and to acquire a uniform thin film layer.
Korean Patent Laid-Open Publication No. 1995-7003746 discloses a method of making a color filter using an inkjet method. However, the resultant color filter suffers similar problems to a color filter made using the dyeing method, for example, deteriorated durability and heat resistance, because this inkjet printing method is performed by also using a dye-type color resist composition and dispersing it from a nozzle to accomplish fine and precise color printing.
Korean Patent Laid-Open Publication Nos. 1993-7000858 and 1996-0029904 disclose an electrophoretic deposition (EPD) method using an electric precipitation method. The electrophoretic deposition (EPD) can form a precise color film having excellent heat resistance and color fastness, since it uses a pigment. However, this method may not be useful in the production of a highly precise color filter requiring a finer electrode pattern for a more precise pixel in the future, because it may produce a colored film that is stained or thicker at both ends due to electrical resistance.
The pigment dispersion method forms a colored film by repeating a series of processes such as coating, exposing to a light, developing, and curing a photopolymer composition including a coloring agent on a transparent substrate including a black matrix.
The pigment dispersion method can improve heat resistance and durability, which are very important characteristics for a color filter, and can provide a film with a uniform thickness. For example, Korean Patent Laid-Open Publications Nos. 1992-7002502, 1994-0005617, 1995-7000359, 1994-0005617, and 1995-0011163 disclose a method of preparing a photosensitive resin composition for a color filter using a pigment dispersion method.
In this pigment dispersion method, a photosensitive resin composition for a color filter generally includes a binder resin, a photopolymerization monomer, a photopolymerization initiator, an epoxy resin, a solvent, and other additives. For example, the binder resin may include a carboxyl-containing acrylic-based copolymer in Japanese Patent Laid-Open Publications Pyung 7-140654 and 10-254133.
The color filter can be subjected to many chemical treatments during the manufacturing process. Accordingly, a color photosensitive resin should have a development margin and chemical resistance sufficient to improve yields of a color filter and to maintain a pattern formed under the aforementioned conditions.
Conventional color liquid crystal displays (LCDs) are generally fabricated by preparing a color filter substrate for displaying a color image separately from an operating substrate on which a thin film transistor (TFT) array is disposed and then binding the color filter substrate and the operating substrate together. However, since there can be low arrangement accuracy during the binding step, conventional color liquid crystal displays can require a shading layer with a large width. Accordingly, it is difficult to increase the aperture ratio (a ratio of an active light-emitting area to a total pixel area). In addition, because the glass substrate and LCD screen have recently become larger, it takes longer for the liquid crystal composition to cover the front side of the substrates during vacuum injection.
A new method has been suggested to sharply decrease the time needed for printing a seal material and dripping a liquid crystal for over-coating but this method has a problem of sharply deteriorated arrangement accuracy.
A method for forming a color filter on the operating TFT array substrate of a TFT color liquid crystal display (LCD) has also been suggested. Since this method does not need a color filter substrate and fabricates a transparent substrate through sputtering and binding two substrates, it has an advantage of simplifying the arrangement and increasing aspect rate.
However, when a color filter is formed on a TFT array substrate, a pixel electrode is formed on the color filter in a photolithography method by using a common positive photoresist. Accordingly, the resist layer needs to be removed after forming the electrode. In other words, a pixel electrode is formed by forming a transparent electrode layer on color pixels of a color filter, coating a positive resist composition thereon, and patterning it, exposing it to light, and developing it. Then, the resist layer remaining on the pixel electrode should be peeled and removed with a resist stripper. Accordingly, the positive resist composition requires resistance against the resist stripper. Conventional photosensitive resin compositions for a color filter, however, typically have poor stripper-resistance.
Conventionally, a pixel electrode is fabricated by forming a transparent layer (a pixel protective layer) having stripper-resistance on a color filter. In addition, a pixel electrode can be fabricated without coating a pixel protective layer by treating a stripper at a low temperature for a longer time to decrease the stripper's influence on a color filter. However, these conventional methods have problems of deteriorating a yield rate and production efficiency, since they require more processes and longer times.
In order to solve these problems, a radiation-sensitive composition with an expansion rate of less than 5% against a stripper to remove a cured layer forming a color layer in a COA method can be used. Further, a color filter can have improved thermal polymerization cross-linking effects by using a multi-functional alicyclic epoxy compound as a thermal polymerization cross-linking agent and a benzophenone-based peroxide as a photo-thermal polymerization initiator.
According to this method, a color filter can be cured at a low temperature for a short time and thereby can have excellent durability and close contacting (adhesion) properties. However, as the demand for larger screens with higher image quality than can be produced using conventional methods increases, there is also an increased need for color filters with a higher aperture ratio and higher performance.